When Ross goes to Rachel's apartment after they watch the tape with the rest of gang, he starts to close the door and you can hear it shut off camera as the scene switches to show Rachel. The camera then cuts back to Ross and he reaches out and closes the door again.
When Ross and Rachel are watching their video, Ross compliments Rachel on her tan and she says she was at the beach that weekend. Since we know their encounter was in April (a month before Chandler and Monica's wedding in May), you have to wonder what beach she's referring to. It would still be chilly in New York in April, if not downright cold.
Whenever Rachel fast-forwards the tape, we hear sped-up audio, which shouldn't happen with a newer VCR (like the one they watch the tape with).
The VCR that they watch the tape on is a Panasonic. The "play" indicator we see on the TV screen is definitely not that of a Panasonic. The remote doesn't look like a Panasonic either.
In The One With the Red Sweater, Joey says he doesn't know who the red sweater belongs to, however it is extremely unlikely, since Ross was wearing the sweater throughout this day and he was with Joey the majority of the episode.
In the flashback scene, Joey is sitting in his chair with Waffle Crisp cereal box, but the W has been altered to read "Vaffle Crisp".
The labels on the water bottles on Monica and Chandler's kitchen table are altered to read Daani instead of Dasani.
When Rachel starts the video tape, none of the cast is reflected in the television but crew members are.
When Ross arrives at Joey's he is only wearing the red sweater, yet after he slept with Rachel that night he left the red sweater behind the following morning.
When Joey learns that Rachel is pregnant, he says there was a dude that spent the night. He says he never saw the guy but he left his sweater behind. Joey actually saw and spoke with Ross, who was wearing the sweater, the night he spent with Rachel. He should have recognized the sweater as belonging to Ross.